One method for nondestructively evaluating or inspecting mechanical materials, components, systems and the like is through measurement of acoustic energy emitted by a specimen under test when it is placed under loads that simulate what the specimen may be expected to experience in use. The load produces strains in the specimen that are relieved by stress waves produced at early failure sites. These stress waves propagate as surface elastic waves which may be sensed as impulsive "sounds" of broad bandwidth. The acoustic emissions are detected by electroacoustic transducers closely coupled to the specimen.
Another nondestructive evaluation technique is photoacoustic spectroscopy. In this case, a laser pulse is absorbed at the surface of the specimen. The surface is heated and its thermal expansion leads to a surface elastic wave. The instant invention differs by the use of much longer wavelength, by a factor 10.sup.5 or 10.sup.6 that permits sub-surface penetration of the composite material specimen. As a result, heat sources are generated in the depth of material and stress is developed over the volume of the specimen, rather than primarily on the surface.